1. FIELD OF THE INVENTION
The present invention relates to an optical scanning apparatus, and more particularly to an optical scanning apparatus suitable for a laser printer that is used as an apparatus for printing out information.
2. DESCRIPTION OF THE PRIOR ART
In laser printers, the laser beam has been deflected to scan a light sensitive drum by the combination of an F.theta. lens and a rotary polygonal mirror. One of problems that would be encountered in scanning the laser beam is that there would be generated a non-uniformity in scanning pitch due to a tilt of reflective surfaces of the rotary polygonal mirror. In order to obviate this problem, there have been proposed various methods for reducing adverse affects due to the tilt error of the rotary polygonal mirror by means of the combination of a cylindrical lens and a toric F.theta. lens (Japanese Patent Unexamined Publication No. 48-98844), the combination of a prism and an F.theta. lens (Japanese Patent Examined Publication No. 59-9883), the combination of a toric lens and an F.theta. lens (Japanese Patent Unexamined Publication No. 48-49315), the combination of a cylindrical lens having at least one of a modified cylindrical surface and an spherical surface where a radius of curvature is varied in surface tilt direction in accordance with a height from an optical axis and an F.theta. lens (Japanese Patent Unexamined Publication Nos. 61-120112 and 61-175607) and the like. However, these methods suffer from difficulties in which the increased number of components must be used due to the use of two kinds of optical elements such as an asymmetrical optical element and a F.theta. lens as described above. In order to overcome these difficulties, it has been also proposed to impart the surface tilt correction function to the F.theta. lens. According to this proposal, there is provided a cylindrical surface (Japanese Patent Unexamined Publication No. 54-126051) or a toric surface (Japanese Patent Unexamined Publication No. 57-144515) as well as ordinary spherical surfaces or planar surfaces. Assuming that a first plane be perpendicular to a second plane, both the planes being intersected with the lens surface, the "toric surface" means a surface having different radii of curvature of the first and second intersection lines. Namely, the toric surface is expressed by the following secondary expansion: EQU z=Ax.sup.2 +By.sup.2 . . . (1)
where z corresponds to the optical axis, xz and yz correspond to the surfaces that are perpendicular to each other and include the optical axis, and A and B are coefficients and are independent of the view angle .theta. relative to the optical axis (A.noteq.B). In the above-described proposal, the focal length (fy) within a surface yz that affects the surface tilt is shorter than the focal length (fx) within the optical scanning surface xz. Thus it is difficult to keep the imaging characteristics in the surface tilt direction in a good condition over the whole scanning region.